The disclosed invention generally relates to an enhancement system for stereo sound reproduction systems, and is particularly directed to a stereo enhancement system which broadens the stereo sound image, provides for an increased stereo listening area, and provides for perspective correction for the use of speakers or headphones.
As is well known, a stereo sound reproduction system attempts to produce a sound image wherein the reproduced sounds are perceived as emanating from different locations, thereby simulating the experience of a live performance. The aural illusion of a stereo sound image is generally perceived as being between the speakers, and the width of the stereo image depends to a large extent on the similarity or dissimilarity between the information respectively provided to the left and right speakers. If the information provided to each speaker is the same, then the sound image will be centered between the speakers at "center stage." In contrast, if the information provided to each speaker is different, then the extent of the sound image will spread between the two speakers.
While the general concept of stereo sound imaging is not complex, its use and implementation is more difficult. The width of the stereo sound image depends not only on the information provided to the speakers, but also on listener position. Ideally, the listener is equidistant from the speakers. With many speaker systems, as the listener gets closer to one speaker, the sound from the more distant speaker contributes less to the stereo image, and the sound is quickly perceived as emanating only from the closer speaker. This is particularly so when the information in each speaker is not very different. However, even with the listener equidistant from the speakers, the perceived sound image is generally between the physical locations of the speakers and does not extend beyond the region between the speakers.
Some known speaker systems have been designed to reduce the limitation that a listener should ideally be located equidistant between speakers. However, such speaker systems are generally complex and the resulting stereo image is still limited to the region between the physical locations of the speakers.
Another consideration in stereo reproduction is the fact that the sound transducers (typically speakers or headphones) are located at predetermined locations, and therefore provide sound emanating from such predetermined locations. However, in a live performance, the perceived sound may emanate from many directions as a result of the acoustics of the structure where the performance takes place. The human ears and brain cooperate to determine direction on the basis of different phenomena, including relative phase shift for low frequency sounds, relative intensity for sounds in the voice range, and relative time arrival for sounds having fast rise times and high frequency components.
As a result of the predetermined locations of speakers or headphones, a listener receives erroneous cues as to the directions from which the reproduced sounds are emanating. For example, for speakers located in front of the listener, sounds that should be heard from the side are heard from the front and therefore are not readily perceived as being sounds emanating from the sides. For headphones or side mounted speakers, sounds that should emanate from the front emanate from the sides. Thus, as a result of the placement of speakers or headphones, the sound perspective of a recorded performance is incorrect.
There have been numerous attempts to spread and widen the stereo image with mixed results. For example, it is known that the left and right stereo signals may be mixed to provide a difference signal (such as left minus right) and a sum signal (left plus right) which can be selectively processed and then mixed to provide processed left and right signals. Particularly, it is well known that increasing or boosting the difference signal produces a wider stereo image.
However, indiscriminately increasing the difference signal creates problems since the stronger frequency components of the difference signal tend to be concentrated in the mid-range. One problem is that the reproduced sound is very harsh and annoying since the ear has greater sensitivity to the range of about 1 KHz to about 4 KHz within the mid-range (herein called the "difference signal components of greater sensitivity"). Another problem is that the listener is limited to a position that is equidistant between speakers since the mid-range includes frequencies having wavelengths comparable to the distance between a listener'ears (which have frequencies in the range of between about 1 KHz and 2 KHz). As to such frequencies (herein called the "difference signal frequency components of increased phase sensitivity"), a slight shift in the position of the listener's head provides an annoying shift in the stereo image. Moreover, the perceived widening of the stereo image resulting from indiscriminate boosting of the difference signal is small, and is clearly not worth the attendant problems.
Some known stereo imaging systems require additional amplifiers and speakers. However, with such systems, the stereo image is limited by the placement of the speakers. Moreover, placing speakers at different locations does not necessarily provide the correct sound perspective.
With other systems, fixed or variable delays are provided. However, such delays interfere with the accuracy of the reproduced sound since whatever delays existed in the performance that was recorded are already present in the recording. Moreover, delays introduce further complexity and limit the listener's position.
There have also been attempts to correct or compensate the improper sound perspective resulting from the use of headphones. However, considerations with known headphone enhancement systems include complexity and lack of effectiveness.